Fluid apparatus



June 27, 1967 r BUDZICH 3,327,642

FLUID APPARATUS Filed Feb. 11, 1965 2 Sheets-Sheet 1 FIG! I N V EN TOR.

7405052 BUflZ/CA/ K70 J MOm June 27, 1967 T. BUDZICH FLUID APPARATUS 2 Sheets-Sheet 2 Filed Feb. 11, 1965 IN V EN TOR.

' 8) MCA/EA/A/K rAwp/A/araq QwpA/s ,s aawam/ .MV/VMLWM United States Patent 3,327,642 FLUID APPARATUS Tadeusz lfiudzich, 80 Murwood Drive, Moreland Hills, Ohio 44022 Filed Feb. 11, 1965, Ser. No. 431,896 3 Claims. (Cl. 103-173) The present invention relates generally to fluid pressure energy translating devices of the piston pump or motor type, and mort particularly to fluid piston pumps and motors employing a stationary cylinder barrel and a rotary cam plate driving a rotary valve plate which sequentially conducts fluid to and from multiple cylinders in the stationary cylinder barrel.

A principal object of this invention is to provide a novel variable displacement axial piston pump or motor in which the effective displacement of the pump or motor is altered by changing the angular relationship between the rotary valve plate and the rotary cam plate.

Another object of this invention is to provide in a variable displacement axial piston pump or motor a novel mechanism permitting the change in angular relationship between the cam plate and the valve plate while maintaining the valve in a state of hydrostatic balance.

Another object of this invention is to provide a novel variable displacement axial piston pump or motor of the type set forth in the preceding objects in which the angular relationship between the rotary valve plate and the rotary cam plate may be varied by linear movement of a control member along the axis of rotation of these members.

Another object of this invention is to provide a novel variable displacement axial piston pump or motor as set forth in the preceding object in which the movement of the control member may be controlled by the use of an expansible chamber fluid motor.

Still another object of this invention is to provide a novel variable displacement pump or motor of the type set forth in the preceding objects in which the angular relationship between the rotary valve plate and the rotary cam plates is effected by means of a ball screw mechanism interconnecting the valve plate and cam plate so that by linear actuation of the screw, the resulting rotation of the nut will produce the change in angular relationship.

Certain inherent advantages of the axial piston pump and motor employing a stationary cylinder barrel and revolving valve plate have been recognized for some time. These advantages include the absence of the disturbing centrifugal couple trying to upseat the cylinder barrel from the valve plate and reduction in throttling losses in the passages of the timing mechanism.

Heretofore, a particular disadvantage of the stationary cylinder barrel has been that it is very difiicult to produce a satisfactory solution of the stroke changing mechanism. The present invention provides a simple, cheap, accurate and reliable method of changing the angular relationship between the cam plate and the valve plate, in a gradual fashion, thus obtaining perfect control over displacement of the unit. This mechanism, working at minimum friction level, may be used in a control of a pressure compensated pump, where the volume output must change proportionally to discharge pressure. In addition, this mechanism besides providing a simple and reliable Way of changing the angular relationship between the cam plate and valve plate permits the change in direction of rotation with unit acting as a motor.

' As it is well known in the art with the principal axis of the cam plate coinciding with the line dividing the valve plate into low pressure and high pressure zones, the pump or motor is in its full displacement, handling the maximum volume of hydraulic fluid. An angular rotation between the principal cam axis and the valve plate will gradually decrease the displacement of the unit, until the volume output becomes zero once 90 relationship is reached. Continued rotation of the axis past 90 Will gradually start increasing the displacement of the unit from zero to maximum reversing the direction of rotation of the unit.

Further objects and advantages of the invention will become apparent from a study of the following specification when taken in conjunction with accompanying drawings in which:

FIGURE 1 is a sectional elevation of a motor according to the preferred embodiment of this invention.

FIGURE 2 is the end view of the cylinder barrel taken along the line 22 and showing the detail of the cylinder barrel porting.

FIGURE 3 is an outside view of the valve plate taken along the line 33 and showing the port details.

The embodiment shown in the drawings Will be described as a motor, although it can work equally well as a pump. As shown in FIGURE 1 a motor housing 10 and a motor cover 11 are suitably bolted together with bolts 12. The motor housing 10 is equipped with a low pressure port 13 and a high pressure port 14 which are connected by suitable means do drain and to a source of high pressure. hydraulic fluid. The fiat inner end bore of the motor body 10 provides a working surface for a rotary valve plate 15. The rotary valve plate 15' Works between this fiat face and the end face of a cylinder barrel 16. The cylinder barrel 16 is equipped with a number of cylinder bores 17 in which pistons 18 are arranged for reciprocation. The reciprocating motion is transmitted to pistons 18 from a reaction plate 19, by articulated connecting rods 20, suitably retained in the reaction plate 19 and pistons 18. A cam plate 21, working in operational contact with the reaction plate B, through an antifriction bearing 22, is located in the motor cover 11 by another antifriction bearing 23. A cylindrical extension 24, of the cam plate 21, is equipped with splines and sealed by a conventional type seal 26. A radially extending guide bearing 27, free to slide in a longitudinal slot 28, permits the wobbling motion of the reaction plate 19, while maintaining the angular relationship between the articulated connecting rods 20 and pistons 18. The cylinder bores 17 of the cylinder barrel 16 are equipped with kidney shaped ports 29 (see FIGURE 2) which work in operational connection with high pressure inlet kidney slots 30, provided in the valve plate 15 (see FIGURE 3). The rotary valve plate 15 is equipped with identical kidney shaped inlet slots 30 and 31, which are connected through a drilled radial passage 32, with space 33, located in the center of the valve plate 15. The drilled passage 32, at the outer end is suitably closed by a plug 34. The cylinder bores 17, of the cylinder barrel 16, on the low pressure stroke are in direct communication with open slot which through passages 36 is connected to the chamber 37 inside the motor housing 10. The chamber 37 is in direct communication with the low pressure or drain port 13. The rotary valve plate 15 on the side away from cylinder barrel 16 works in operational contact with end 38 of a transfer sleeve 39. On the other side the valve plate 15 works in operational contact with sealing face 40 of a hollow balancing member 41. A spring 42 is located between the motor housing 10 and the transfer sleeve 39. The transfer sleeve 39 is suitably sealed by an O-ring 43. The hollow balancing member 41 is connected to a balancing piston 44 by a snap ring 45' and sealed by O-ring 46. The balancing piston 44 is equipped with two sealing O-rings 48 and 49. A longitudinal passage 50 in member 41 connects inner chamber 51 with outer chamber 52. Space 53 is enclosed between the stepped diameters of balancing piston 44 and is vented to atmosphere by suitable passage 54. A control cylinder member 55 is non-rotatably connected through flange 56 to the rotary valve plate 15 by dowel pins 57 and suitable bolts (not shown). The circular extension of the control cylinder member 55 encloses inner chamber 51 connected through passage 50 with outer chamber 52 and control port 58. The outer surface of the control cylinder member 55 works in operational contact with the inside bore of cylinder barrel 16 and locates it with respect to rotary valve plate 15. A control piston 59 works in bore 60 of the control cylinder 55 and with its cylindrical extension 61 rests against flat face of a control spring guide 62. Slots 63, machined in the end of the control cylinder 55, slidably receive projecting ears 64- on the control spring guide 62, which in turn is fastened by a pin 65 to the control ball screw 67. The control ball screw 67 engages, through a series of balls 68, control ball nut 66, permanently retained in the cam plate 21. A compression spring 69 is located between the control ball nut 66 and control spring guide 62. The cylinder barrel 16 with its spherical surface 70 engages the ring 71 and is secured against rotation by pin 72 engaging an oval slot 73. The oval slot 73 permits the cylinder barrel 16 to align itself to the rotary valve plate 15 while preventing it from rotating.

The housing with its ring 71 engages the spherical surface 70 of the cylinder barrel 16. The cylinder barrel 16, equipped with kidney shaped ports 29, works in operational contact with rotary valve plate 15. As rotary valve plate revolves between the flat faces of cylinder barrel 16 and housing 10, the kidney shaped inlet slots 30, located on both faces of the valve plate 15, sequentially register with the kidney shaped slots 29, conducting the high pressure fluid, supplied to and from the cylinder bores 17 through the ports 29, space 33, space 75 and port 14. Slot 35, provided in the rotary valve plate 15, sequentially registers with cylinder bores 17 through kidney shaped ports 29, conducting the low pressure oil through a series of passages 36 to chamber 37 and port 13.

The cylinder barrel 16 is prevented from rotating by pin 72 and while pivoting around the spherical surface 70 is free to align itself, under action of unbalanced hydrostatic forces, against the flat face of the rotary valve plate 15. Since both flat surfaces of the rotary valve plate 15 are identical, the unbalanced force coming from the cylinder barrel 16 will effectively seal both rotary surfaces of the valve plate, permitting operation of the pump or motor at a minimum leakage level. The pin 72 works in oval slot 73 and permits not only a measure of universal freedom of the cylinder barrel 16 but also a measure of axial freedom, permitting the valve plate to work with minimum end clearances. The pistons 18 are mounted for reciprocation in cylinder bores 17 of the cylinder barrel 16 and are connected by articulated connecting rods to the reaction plate 19, while being universally mounted at their spherical ends. The reaction plate 19, located on the antifriction bearing 22, interposed between the reaction plate 19 and the cam plate 21, is prevented from rotation by the bearing 27, working in slot 28, while being permitted to transmit the wob bling and therefore reciprocating motion through the articulated connecting rods 20 to pistons 18. The cam plate 21 is interposed between the reaction plate 19 and the end cover 11. The antifriction bearing 23 is positioned between the cam plate 21 and cover 11. The rotary motion is transmitted to the cam plate 21 by the cylindrical extension 24, sealed by the seal 26 and equipped with spline 25, suitably connected to coupling of driving or driven source.

Between the cam plate 21 and the valve plate 15 a mechanism is interposed with can change the angular relationship of the cam plate 21 and the valve plate 15, while transmitting the rotary motion from one to the other. This mechanism is basically composed of the control ball nut 66 .which is secured to the cam plate 21 on its axis of rotation. The control ball nut 66, through a series of balls 68, operationally engages the control ball screw 67 which is secured to the control spring guide 62 which in turn, through the projecting ears 64, engages slot 63 of the control cylinder 55, permanently secured by dowel pin 57 and bolts, not shown, to the valve plate 15. The control spring 62 is interposed between the ball nut 66 and the spring retainer 62 and provides the reaction which permits the transmissionrof rotary motion between the cam plate 21 and the valve plate 15.

With he unit working as a motor, fluid introduced to high pressure port 14 is conducted through space 33 and drilled passage 32 to kidney shaped port 29 and cylinder bore 17. The resultant hydrostatic forces, developed in the cylinder bores 17, maintain cylinder barrel 16 against flat face of the rotary valve 15, which is kept by the same force in abutment with working surface of the housing 10. The non-rotating cylinder barrel 16, equipped with spherical surface 70 working in contact with ring 71, is capable of aligning itself to the surface of the rotary valve plate 15 containing the timing slots and therefore of working at minimum leakage level. The fluid under pressure, contained in the cylinder bore 17, reacts against projected area of piston 18. This creates a force which is transmitted through universally mounted connecting rods 20 to the reaction plate 19, which is stabilized against rotation by thebearing 27 engaging slot 28, and through series of balls 22 to the cam plate 21. The sum of the hydraulic forces, transmitted from the pistons subjected to pressure, acting on the inclined plane of the cam plate 21, will induce rotation. The rotary motion of the cam plate 21 is transmitted to the rotary valve plate 15 by the displacement changing mechanism. The exhaust'fluid from the cylinder bores 17, at low pressure, is displaced through kidney shaped ports 29, open slot 35 and the series of passages 36 provided in the rotary valve plate 15, to chamber 37 and low pressure port 13. The cam plate 21 and valve plate 15 revolve under action of the torque generated on the inclined surface of the cam plate. During rotation the fluid under pressure is sequentially introduced by the rotary valve plate 15 to cylinder bores 17 of stationary selfaligning cylinder barrel 16. The exhaust oil is conducted by the above described passages contained in the rotary valve plate to low pressure port 13. The actual torque developed in the motor will depend on the magnitude of the pressure of the fluid introduced to the cylinder bores 17 and on the angular relationship between the inclined surface of the cam plate 21 and the rotary valve plate 15. With the principal axis of the cam passing through the highest and lowest points of the inclined surface, coinciding with the axis dividing rotary valve plate 15 into the high and low pressure Zones, the maximum torque is generated, the displacement of the ,unit per one revolution being at its greatest. When the angular relationship between these two axes is changed from this maximum position in either direction, a gradual reduction both in torque and displacement of the unit will result until a point is reached when with axis angularly disposed by where both the torque output and effective displacement of the motor will become zero. Further angular rotation of the axis past the 90 position will gradually increase the torque output and the displacement of the motor while changing the direction of rotation of the output shaft. The present invention is mainly concerned with a mechanism which will change the angular relationship between the cam plate and the valve plate and which is responsive to a controllable signal whereby the motor displacement, torque output and direction of rotation can be controlled.

The mechanism transmitting the driving torque and con trolling the angular relationship between the rotating cam plate 21 and valve plate 15 is based on the principle of inclined plane where any relative rotation between these two members must be accompanied by proportional relative axial movement within the mechanism. The ball nut 66 engages the ball screw 67 through the series of balls 68 working at minimum friction level so that the unit is reversible. With the ball nut 66 secured to cam plate 21 the ball screw 67 slidably engages with cylindrical pins 64, slots 63, provided in the control cylinder 55 which is secured to the rotary valve plate 15. The controlled axial movement of the ball screw 66 will produce a proportional change in the angular relationship between the cam plate 21 and the valve plate 15, thus changing the efiective displacement and the torque output of the fluid motor.

Since the stroke changing mechanism must transmit torque equivalent to frictional resistance of the rotary valve plate 15, an axial force must be supplied to maintain the ball screw 67 in any specified position. This force must be equal to the reaction of the transmitted torque multiplied by mechanical advantage of the control ball screw 67. With the control mechanism at rest the control spring 69 maintains the control ball screw 67 in its extreme position against a stop, provided by the control cylinder 55, as shown in FIGURE 1. In this position the angular relationship between the cam plate 21 and valve plate is such that the fluid motor is capable of developing maximum torque at maximum eflective displacement for to tation of the cam plate 21 in one direction. The preload in the control spring 69 and therefore the force exerted on the ball screw 67 is so selected that the maximum torque can be supplied to the valve plate 15, without producing a relative rotation of the control mechanism.

The action of the control spring 69 is opposed by a hydraulic force acting on the control piston 59. Fluid under controlled pressure is supplied from port 58 and longitudinal passage to chamber 51 which is in direct communication with bore of the control cylinder 55. When the hydraulic force, induced by control pressure acting on the control piston 61, will balance either a sum or diflerence of axial force torque component and preload in the control spring 69, depending on the direction of rotation, the control mechanism will be in a state of equilibrium. Any further increase in the control pressure beyond this point will move the control ball screw 67 from its extreme position, compressing the control spring 69.

Still further increase in the control pressure will produce a movement of the control ball screw 67 and corresponding increase of the phase shift between the cam plate 21 and the valve plate 15, the magnitude of the increase depending on the rate of the control spring 69. With a continuing increase in control pressure level, at a phase shift of eflective displacement and theoretical torque output of the fluid motor will become zero. Further movement of the control ball screw 67 will reverse the direction of rotation of the motor, gradually bringing its effective displacement and torque to the maximum value when reaching phase shift. With the change of rotation of the fluid motor at phase shift of 90 the axial component of the friction torque, transmitted through the control mechanism, will change its direction, helping instead of opposing the force exerted by the control spring 69. This will result in a higher increase of control pressure per unit length of travel of the control ball screw 67. The change in direction of rotation will always take place with fluid motor at rest, thus eliminating shock loads. In this way torque output, effective displacement and direction of rotation of the motor can be controlled by a variable pressure signal.

The influence of the control pressure on the hydrostatic balance of the valve plate 15 is eliminated by the introduction of fluid at control pressure to chamber 52, the effective area of the enlarged end of the hollow balancing member 41 being the same as the area exposed to pressure in space 52. The effective areas of the hollow balancing plunger 41, the transfer sleeve 39 and the smaller end of the balancing piston 44 subjected to motor inlet pressure and pressure gradient on the sealing surfaces are so arranged that the hydraulic forces cancel each other, valve plate 15 remaining in state of perfect equilibrium, with its sealing surfaces hydrostatically balanced. An introduction of a device in form of a mechanical link spaced in the longitudinal passage 50 and engaging the control piston 59 will produce a mechanically operated control,

6 where the position of control ball screw 67 can be changed at will.

While I have illustrated and described a preferred form of my invention and some modifications thereof it will be apparent to those skilled in the art that the device is capable of various additional modifications without departing from the scope and spirit of the invention as defined in the following claims.

What is claimed is:

1. An energy translating fluid pressure device comprising a housing defining a fluid chamber therein, a first supply port on said housing opening into said chamber, a cylinder barrel nonrotatably mounted within said chamber and having a plurality of cylinder bores extending axially therein, a cam plate journaled in said housing within said chamber for rotation about an axis parallel to said cylinder bores, a piston within each of said cylinder bores, means operable by said cam plate to progressively reciprocate said pistons, a valve plate mounted in said housing for rotation about the axis of said cam plate, said valve plate being mounted between the end of said cylinder barrel opposite said cam plate and said housing, said cylinder barrel being free for limited movement toward and away from said valve plate and free to align itself with said end of said cylinder barrel against said valve plate, said cylinder barrel being shaped so that pressure in said cylinder bores produces a hydrostatic force urging said cylinder barrel against said valve plate with a force which is a direct function of the pressure in said cylinder bores, said valve plate having first and second ports arranged to be sequentially moved into registration with said cylinder bores as said valve plate is rotated, said first valve plate port being in communication with said fluid chamber and said first supply port, drive means interconnecting said cam plate and said valve plate, said driving means including an antifriction screw and nut assembly, said nut being fixedly secured to said cam memher, said screw being connected to said valve plate by a spline allowing said screw to move axially with respect to said valve plate member and said nut and rotatable with said valve plate, whereby axial movement of said screw changes the phase relationship between said valve plate and said cam plate to control the effective displacement of said device, an expansible chamber control motor including a cylinder member carried by said valve plate and a control piston slidable within said cylinder member and engageable with said screw to shift said screw toward said cam plate, spring means to shift said screw away from said cam plate, an axial passage in said valve plate, said passage being connected to the other of said valve plate ports, a second supply port on said housing, a longitudinal passage in said housing co-axial with the axis of rotation of said valve plate and extending between said valve plate and said second supply port, a sleeve slidable in said axial passage and adapted to make sealing connection between said second supply port and the axial passage of said valve plate whereby fluid may flow between said second valve plate port and said second supply port, a transfer tube mounted within said axial housing passage and said valve plate passage, said transfer tube making sealing contact with said valve plate to admit control fluid to said control motor, and control port means on said housing adapted to admit fluid pressure into said transfer tube, the hydrostatic pressures on said valve plate being substantially balanced.

2. An energy translating fluid pressure device comprising a housing defining a fluid chamber therein, a first supply port on said housing opening into said chamber, a cylinder barrel nonrotatably mounted within said chamber and having a plurality of cylinder bores extending axially therein, a cam plate journaled in said housing within said chamber for rotation about an axis parallel to said cylinder bores, a piston within each of said cylinder bores, means operable by said cam plate to progressively reciprocate said pistons, a valve plate mounted in said housing for 7 rotation about the axis of said cam plate, said valve plate being mounted between the end of said cylinder barrel opposite said cam plate and said housing, said cylinder barrel being free for limited movement toward and away from said valve plate and free to align itself with said end of said cylinder barrel against said valve plate, said cylinder barrel being shaped so that pressure in said cylinder bores produces a hydrostatic force urging said cylinder barrel against said valve plate with a force which is a direct function of the pressure in said cylinder bores, said valve plate having first and second ports arranged to be sequentially moved into registration with said cylinder bores as said valve plate is rotated, said first valve plate port being in communication with said fluid chamber and said first supply port, drive means interconnecting said cam plate and said valve plate, said driving means including an antifriction screw and nut assembly, said nut being fixedly secured to said cam member,said screw being connected to said valve plate by a spline allowing said screw to move axially with respect to said valve plate member and said nut and rotatable with said valve plate, whereby axial movement of said screw changes the phase relationship between said valve plate and said cam plate to control the effective displace ment of said device, an expansible chamber control motor including a cylinder member carried by said valve plate and a control piston slidable within said cylinder member and engageable with said screw to shift said screw toward said cam plate, spring means to shift said screw away from said cam plate, an axial passage in said valve plate, said passage being connected to the other of said valve plate ports, a second supply port on said housing, a longitudinal passage in said housing co-axial with said valve plate and extending between said valve plate and said second supply port, a sleeve slidable in said axial passage and adapted to make sealing connection between said second supply port and the axial passage of said valve plate whereby fluid may flow between said second valve plate port and said second supply port, a transfer tube mounted within said axial housing passage and said valve plate passage, said transfer tube having a radial flange adapted to make sealing contact against said valve plate on the side adjacent said cylinder barrel, a control chamber on the end of said housing outward of said second supply port, said transfer tube extending from said flange through said valve plate passage, said sleeve and into said control chamber, seal means to prevent fluid communication between said control chamber and said second supply port, a piston on said transfer tube whereby pressures in said control chamber and said hydraulic cylinder on said transfer tube 8 are balanced, and control port means on said housing adapted to admit fluid pressure through said transfer tube into said control motor, the hydrostatic pressures on said valve plate being substantially balanced.

3. An energy translating fluid pressure device comprising a housing defining a cavity, a generally nonrotatable cylinder barrel mounted in said housing within said cavity, said cylinder barrel being formed with a plurality of cylinder bores, pistons recipro-cable in said bores, cam means mounted to rotate in said housing and operatively associated with said pistons, a low pressure port in said housing open to said cavity, a high pressure port in said housing, a valve plate positioned between one end of said cylinder barrel and said housing drive means connected between said cam means and valve plate operating said valve plate to sequentially connect said cylinder bores to said cavity and to said high pressure port, a control pressure port in said housing, said drive means including a fluid motor connected to said control pressure port operable to change the position of said valve plate with respect to said cam means and thereby change the eflective displacement of said device in response to changes in pressure at said control pressure port, a hydrostatically balance transfer tube extending through said valve plate to connect said control pressure port to said fluid motor, the exterior of said transfer tube defining at least part of a passage connecting said valve plate and said high pressure port, said cylinder barrel being mounted in said housing for limited free movement toward and away from said valve plate and so that said end of said cylinder barrel is substantially free to align itself with said valve plate, said cylinder barrel being shaped so that it is hydrostatically urged toward said valve plate by fluid under pressure in said cylinder bores with a force which is a direct function of the pressure in said cylinder bores, said valve plate being hydrostatically balanced so that changes in pressure in said high pressure port and said control pressure port do not produce any substantial resulting forces on said valve plate.

References Cited UNITED STATES PATENTS 2,969,810 1/1961 Dudley 103228. 3,011,453 12/1961 Budzich 103-173 3,074,345 1/1963 SchefioW 103173 DONLEY J. STOCKING, Primary Examiner.

MARK NEWMAN, Examiner.

R. M. VARGO, W. L. FREEH, Assistant Examiners. 

1. AN ENERGY TRANSLATING FLUID PRESSURE DEVICE COMPRISING A HOUSING DEFINING A FLUID CHAMBER THEREIN, A FIRST SUPPLY PORT ON SAID HOUSING OPENING INTO SAID CHAMBER, A CYLINDER BARREL NONROTATABLY MOUNTED WITHIN SAID CHAMBER AND HAVING A PLURALITY OF CYLINDER BORES EXTENDING AXIALLY THEREIN, A CAM PLATE JOURNALED IN SAID HOUSING WITHIN SAID CHAMBER FOR ROTATION ABOUT AN AXIS PARALLEL TO SAID CYLINDER BORES, A PISTON WITHIN EACH OF SAID CYLINDER BORES, MEANS OPERABLE BY SAID CAM PLATE TO PROGRESSIVELY RECIPROCATE SAID PISTONS, A VALVE PLATE MOUNTED IN SAID HOUSING FOR ROTATION ABOUT THE AXIS OF SAID CAM PLATE, SAID VALVE PLATE BEING MOUNTED BETWEEN THE END OF SAID CYLINDER BARREL OPPOSITE SAID CAM PLATE AND SAID HOUSING, SAID CYLINDER BARREL BEING FREE FOR LIMITED MOVEMENT TOWARD AND AWAY FROM SAID VALVE PLATE AND FREE TO ALIGN ITSELF WITH SAID END OF SAID CYLINDER BARREL AGAINST SAID VALVE PLATE, SAID CYLINDER BARREL BEING SHAPED SO THAT PRESSURE IN SAID CYLINDER BORES PRODUCES A HYDROSTATIC FORCE URGING SAID CYLINDER BARREL AGAINST SAID VALVE PLATE WITH A FORCE WHICH IS DIRECT FUNCTION OF THE PRESSURE IN SAID CYLINDER BORES, SAID VALVE PLATE HAVING FIRST AND SECOND PORTS ARRANGED TO BE SEQUENTIALLY MOVED INTO REGISTRATION WITH SAID CYLINDER BORES AS SAID VALVE PLATE IS ROTATED, SAID FIRST VALVE PLATE PORT BEING IN COMMUNICATION WITH SAID FLUID CHAMBER AND SAID FIRST SUPPLY PORT, DRIVE MEANS INTERCONNECTING SAID CAM PLATE AND SAID VALVE PLATE, SAID DRIVING MEANS INCLUDING AN ANTIFRICTION SCREW AND NUT ASSEMBLY, SAID NUT BEING FIXEDLY SECURED TO SAID CAM MEMBER, SAID SCREW BEING CONNECTED TO SAID VALVE PLATE BY A SPLINE ALLOWING SAID SCREW TO MOVE AXIALLY WITH RESPECT TO SAID VALVE PLATE MEMBER AND SAID NUT AND ROTATABLE WITH SAID VALVE PLATE, WHEREBY AXIAL MOVEMENT OF SAID SCREW CHANGES THE PHASE RELATIONSHIP BETWEEN SAID VALVE PLATE AND SAID CAM PLATE TO CONTROL THE EFFECTIVE DISPLACEMENT OF SAID DEVICE, AN EXPANSIBLE CHAMBER CONTROL MOTOR INCLUDING A CYLINDER MEMBER CARRIED BY SAID VALVE PLATE AND A CONTROL PISTON SLIDABLE WITHIN SAID CYLINDER MEMBER AND ENGAGEABLE WITH SAID SCREW TO SHIFT SAID SCREW TOWARD SAID CAM PLATE, SPRING MEANS TO SHIFT SAID SCREW AWAY FROM SAID CAM PLATE, AN AXIAL PASSAGE IN SAID VALVE PLATE, SAID PASSAGE BEING CONNECTED TO THE OTHER OF SAID VALVE PLATE PORTS, A SECOND SUPPLY PORT ON SAID HOUSING, A LONGITUDINAL PASSAGE IN SAID HOUSING CO-AXIAL WITH THE AXIS OF ROTATION OF SAID VALVE PLATE AND EXTENDING BETWEEN SAID VALVE PLATE AND SAID SECOND SUPPLY PORT, A SLEEVE SLIDABLE IN SAID AXIAL PASSAGE AND ADAPTED TO MAKE SEALING CONNECTION BETWEEN SAID SECOND SUPPLY PORT AND THE AXIAL PASSAGE OF SAID VALVE PLATE WHEREBY FLUID MAY FLOW BETWEEN SAID SECOND VALVE PLATE PORT AND SAID SECOND SUPPLY PORT, A TRANSFER TUBE MOUNTED WITHIN SAID AXIAL HOUSING PASSAGE AND SAID VALVE PLATE PASSAGE, SAID TRANSFER TUBE MAKING SEALING CONTACT WITH SAID VALVE PLATE TO ADMIT CONTROL FLUID TO SAID CONTROL MOTOR, AND CONTROL PORT MEANS ON SAID HOUSING ADAPTED TO ADMIT FLUID PRESSURE INTO SAID TRANSFER TUBE, THE HYDROSTATIC PRESSURES ON SAID VALVE PLATE BEING SUBSTANTIALLY BALANCED. 